Subsequent to drilling or workover of a subterranean oil or gas well, it is sometimes desirable to gravel pack same in order to prevent solid particulate matter in consolidated production formations from being co-produced with the fluid hydrocarbons through the production conduit to the top of the well. In such operations, a "pre-pack" well screen may be utilized, along or in conjunction with exterior conventional gravel-packing techniques. In many instances, such gravel packing is performed without use of a "pre-pack" well screen and gravel is circulated in a viscous carrier fluid for deposition around the exterior of the well screen, which is positioned across the production zone. The deposited gravel prevents the solid particulate matter within the fluid hydrocarbons to freely pass therethrough, and the screen prevents the solids forming the gravel pack from entering into the interior of the production conduit, yet permits the fluid hydrocarbons to pass through porous openings therethrough.
In some instances, the gravel packing of a subterranean well is performed by depositing solid particulate matter, i.e., sand, within a highly viscous carrier fluid. This fluid body is introduced through a tubular conduit and placed within the bore across the production zone to straddle the open perforations. Thereafter, the tubing is withdrawn from the well, and the appropriate screen assembly, which may or may not include a "pre-pack" screen, is run into the well and inserted into the viscous body of fluid containing the gravel.
Since many of the carrier fluids are a highly viscous, high molecular weight, polymeric substance, they are typically shear-thinning, thixotropic substances. Typical of such materials is a product marketed by the Kelco Corporation under the name "XC Polymer," which is a bacterial fermentation product of a polysacharide exposed to the bacteria xanthomonas campestris. When such fluid is agitated, its viscosity is reduced. However, when agitation is decreased, or stopped, the rheological property of the material is reversed and it becomes thixotropic, and the viscosity of the fluid increases substantially to permit the fluid to hold the solid particulate matter in suspension.
Due to the high viscosity and thixotropic nature of such fluids, insertion of the well "pre-pack" or other screen through the fluid will be resisted, often causing more torque and/or weight to have to be applied through the length of the drillstring. Additionally, the thixotropic properties of such fluid also contribute substantially to the difficulties in removing any such screen assemblies, thus often requiring considerably more torque to be applied through the tubing.
Such high viscous and thixotropic fluids many times are utilized as completion or "kill" fluids to be placed across the production zone prior to or subsequent to perforating the casing. In such instances, it becomes considerably more difficult to insert the gun through such viscous completion fluids or to easily withdraw same from the fluid subsequent to the perforating step.
The present sequence that is employed involves perforation of the formation using a gun mounted to the end of a tubing string below a retrievable packer. After perforating the formation and allowing the well to flow to clean up the perforations, the packer is released and the well is killed by bullheading or pumping down the tubing into the formation or by reverse circulating down the annulus and up the tubing of kill fluids of sufficient density to keep the well from coming in as the tubing, including the retrievable packer, and the perforating gun are withdrawn completely out of the well. After removing the perforating gun, the tubing is reinserted into the well to facilitate the introduction of sand as part of the gravel-packing procedure. An alternative to removing the perforating gun completely out of the well requires pulling up the perforating gun after it is fired, sufficiently above the perforations so that when sand is delivered down the tubing, the packed sand column will not reach the position of the raised-up perforating gun. In order to raise the perforating gun, the retrievable packer has to be released, which again requires an initial killing of the well by bullheading or reverse circulating as previously described. The introduction of the killing fluids to the newly perforated formation has a negative effect on the productivity of the formation through the perforations. In employing the methodology of raising the gun above the perforation or coming completely out of the hole with the gun prior to the introduction of sand, the formation is exposed to a larger volume of "kill fluids," as well as a portion of the volume in the tubing string which is displaced during the deposition of sand ("squeezing") into the perforations.
As a means of getting around pulling the gun completely out of the hole or pulling it up sufficiently high above the perforations, another alternative would be to leave the gun in place. The problem with past designs of guns has been that the placement of sand with the gun in place adjacent the perforations can result in sticking of the gun at the bottom of the hole as the sand packs around the gun.
Another concern is how well the perforations clean up after the gun is fired. With past designs, the flow velocities in the region where the gun is mounted have been sufficiently slow to prevent comprehensive elimination of debris when the formation starts to flow after the perforating gun is fired.
The placement of a structure to facilitate extraction, such as an auger blade on the gun, allows clean up by initial flowing of the well with the formation isolated. The reversing out using kill fluids, which is carried on thereafter, occurs above the packet without any effect on the newly created perforations. Thereafter, without releasing the packer or moving the gun, the appropriate charge of sand can be spotted via circulation, again with the formation isolated. When the sand is properly spotted, it can then be directed through a ported disc located between the packer and the perforating gun into the newly created perforations caused by firing of the gun. This mechanism allows the placement of sand in the formation with a specifically selected carrier fluid as opposed to commonly used killing compounds. For example, a stimulating fluid can be used to spot the sand such that when the sand is properly spotted, the amount of liquid bullheaded into the formation to place the sand in the perforations can be a limited quantity of the most beneficial fluid to promote efficient flow of hydrocarbons from the formation through the newly made perforations created by shooting off the gun.
The auger blade around the perforating gun, which straddles the openings in the perforating gun so as not to be damaged by shooting off the gun, creates several advantages. After the formation is perforated and begins to flow, the flights of the auger create a tortuous path, thereby increasing the velocity of the gases and/or liquids produced from the formation. This increased velocity promotes the removal of the debris generated from firing the gun. Additionally, the positioning of the auger blades on the outside of the perforating gun facilitates the removal of the gun, even after the sand is pumped into the perforations. The string can be merely lifted and/or simultaneously rotated and the addition of the flights allows the gun to avoid getting stuck in the compacted sand at the newly packed perforations. in essence, the only resistive force against removing the gun from the sand is the weight of the sand accumulated between the flights of the auger. To the extent necessary, a rotational force can be applied to the gun to facilitate its removal in case of sticking. In the preferred embodiment, the auger is disposed in a manner such that rotation of the drillstring to tighten up its components results in a counter-rotation of the flights of the auger to assist in breaking loose from any obstruction as the gun is removed from the sand. The auger can be left- or right-handed without departing from the spirit of the invention.
With the advent of directional drilling, formations are now perforated in ever increasing lengths. Rather than having a perforating gun extending for about 20-50 feet, assemblies of perforating guns for deviated wellbores are now in use where the perforation takes place over a much longer length of wellbore. Lengths of 3,000-4,000 feet are not unusual. In many cases, the formation which being perforated is unconsolidated. This requires the wellbore to be cased prior to perforation to avoid collapse of the wellbore upon perforation. This has additionally created the need for a better way to remove perforation debris because wells with longer lengths of perforated formation produce a greater volume of debris than typical vertical wells. It has also enhanced the concerns about removal of the perforating gun in view of the extremely long lengths of gun placed in the wellbore.
In the past, where gun lengths have been short, it has been satisfactory to circulate fluids or reverse circulate fluids using the tubing in the annulus to remove debris. However, the low point of the circulation flowpath is above the top of the gun. Accordingly, the debris generated in the immediate proximity of the perforating gun is not effectively removed by circulation or reverse circulation.
Additional problems are created by small depth deviated wellbores where the zone to be perforated is nearly horizontal and extends for a significant length. Using existing available perforating equipment, kill fluids cannot be brought close to the newly formed perforations because of the location of the low point in the circulation or reverse circulation flowpath. The inability to get heavy weight brines or other kill fluids into the immediate viciniy of a perforation which extends several thousand feet, albeit at a low depth from the surface, can present a significant problem. The possibility exists that the well may actually come in during the efforts to circulate out the debris. This can occur because an insufficient weight of heavy fluid bears down on the newly made perforations in shallow deviated wells.
The apparatus of the present invention has been developed to assist in dealing with some of these problems. To facilitate the removal of debris after perforation, the perforating gun or another tool run in subsequent to the perforating of the wellbore can have a tortuous path formed on its outer periphery, or any other mechanism or projection which will increase the flowing velocity. An auxiliary source of fluid can be applied from the surface or from within the wellbore, internally through the tool or perforating gun or adjacent its exterior periphery, or if the tortuous path is formed from a hollow-flight auger, the auxiliary fluid can be added from the surface or from within the wellbore through the auger flights, and extending to a predetermined depth of the auger. While the energy of the formation fluids when the well is allowed to flow is used in combination with the tortuous path to pick up the debris generated in perforation, the use of the auxiliary fluid further increases velocity and the ability to efficiently remove the generated debris.
Augers have previously been applied to screens, as illustrated in U.S. Pat. Nos. 2,513,944; 1,080,684; and 2,371,391. Also cited as relevant to the general field of tubing-conveyed perforating and sand control are U.S. Pat. Nos. 4,681,163; 2,336,586; and manuals put out by Baker Sand Control, a Baker Hughes company, regarding perforating systems, entitled "Tubing-Conveyed Perforating Systems," as well as a manual on gravel-packed systems put out by Baker Sand Control entitled "Products, Services and Accessories."